1. Field of the Invention
This invention relates to castable composite rocket propellants and plastic bonded explosive compositions. More particularly, this invention relates to energetic compositions containing an improved polyalkylene oxide binder.
2. Description of the Prior Art
Elastomeric binders are used in desensitizing energetic compositions. Binders that contribute to enhanced toughness have been found to improve the general hazard sensitivities of high energy rocket propellants. An increase in toughness, as measured by tensile strength and elongation, of plastic-bonded explosives will decrease the hazard sensitivities particularly to those stimuli which cause an increase in the surface area.
Hydroxyl-terminated polyalkylene oxides having a molecular weight of about 4500 and a functionality of 2 are known in the art of elastomeric binder formulations used to produce propellents and explosives. Difunctional polyalkylene oxide polymers with longer chain lengths aparently do not enhance toughness because the cross-link density becomes too low and the compositions become excessively soft. It is also known that cross-linking of the polyalkylene oxide improves the mechanical properties of the binder. Because of the low functionality of the polyether binder material it is necessary to use isocyanate curatives having functionalities greater than 2 in order to obtain adequate cross linking of the polyethers. Such multifunctional isocyanate curatives cannot be obtained as pure compounds and often vary in quality. These variations and impurities adversely affect the mechanical properties and the reliability of the propellant binder formulations.
Polyalkylene oxide triols having molecular weights from about 1000 to about 2400 are known as cross-linkers for propellant binder compositions consisting essentially of difunctional polyalkylene oxides of the same molecular weight. Similarly, polyalkylene oxide triols having a molecular weight between about 3500 and 4500 are known as cross-linkers for propellant binder compositions consisting essentially of difunctional hydroxyl-terminated polybutadienes of the same molecular weight. Formulations containing these low to moderate molecular weight polyalkylene oxide diols and triols have tensile strengths of 75 to 100 psi or more and elongations at maximum stress of over 300%. Onset of volume dilatation occurs in these formulations at levels of strain of about 3% to about 7%.
Castable high energy composite rocket propellant compositions and plastic bonded explosive compositions contain high levels of plasticizer not only to enhance energy but also to improve rheological properties during processing, to prevent crystallization after curing, and to improve low temperature properties of the cured compositions. However, high levels of plasticizers weaken binder strength. Consequently, binder formulations having improved mechanical properties are needed to offset the weakening effects of high plasticizer levels.